A Digital Television Standard published Sep. 16, 1995 by the Advanced Television Subcommittee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals in 6-MHz-bandwidth television channels such as those currently used in over-the-air broadcasting of National Television Subcommittee (NTSC) analog television signals within the United States. The VSB DTV signal is designed so its spectrum is likely to interleave with the spectrum of a co-channel interfering NTSC analog TV signal. This is done by positioning the pilot carrier and the principal amplitude-modulation sideband frequencies of the DTV signal at odd multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal that fall between the even multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal, at which even multiples most of the energy of the luminance and chrominance components of a co-channel interfering NTSC analog TV signal will fall. The video carrier of an NTSC analog TV signal is offset 1.25 MHz from the lower limit frequency of the television channel. The carrier of the DTV signal is offset from such video carrier by 59.75 times the horizontal scan line rate of the NTSC analog TV signal, to place the carrier of the DTV signal about 309,877.73 kHz from the lower limit frequency of the television channel. Accordingly, the carrier of the DTV signal is about 2,690122.27 Hz from the middle frequency of the television channel. The exact symbol rate in the Digital Television Standard is (684/286) times the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The number of symbols per horizontal scan line in an NTSC analog TV signal is 684, and 286 is the factor by which horizontal scan line rate in an NTSC analog TV signal is multiplied to obtain the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The symbol rate is 10.762238 megasymbols per second, which can be contained in a VSB signal extending 5.381119 MHz from DTV signal carrier. That is, the VSB signal can be limited to a band extending 5.690997 MHz from the lower limit frequency of the television channel.
The ATSC standard for digital HDTV signal terrestrial broadcasting in the United States of America is capable of transmitting either of two high-definition television (HDTV) formats with 16:9 aspect ratio. One HDTV format uses 1920 samples per scan line and 1080 active horizontal scan lines per 30 Hz frame with 2:1 field interlace. The other HDTV format uses 1280 luminance samples per scan line and 720 progressively scanned scan lines of television image per 60 Hz frame. The ATSC standard also accommodates the transmission of DTV formats other than HDTV formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal.
DTV transmitted by vestigial-sideband (VSB) amplitude modulation (AM) during terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments. There are 832 symbols per data segment. So, with the symbol rate being 10.76 MHz, each data segment is of 77.3 microseconds duration. Each segment of data begins with a line synchronization code group of four symbols having successive values of +S, -S, -S and +S. The value +S is one level below the maximum positive data excursion, and the value -S is one level above the maximum negative data excursion. The initial line of each data field includes a field synchronization code group that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511-sample pseudo-noise sequence (or "PN-sequence") followed by three 63-sample PN sequences. This training signal is transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one's complementary respective to each other.
The data within data lines are trellis coded using twelve interleaved trellis codes, each a 2/3 rate trellis code with one uncoded bit. The interleaved trellis codes are subjected to Reed-Solomon forward error-correction coding, which provides for correction of burst errors arising from noise sources such as a nearby unshielded automobile ignition system. The Reed-Solomon coding results are transmitted as 8-level (3 bits/symbol) one-dimensional-constellation symbol coding for over-the-air transmission, which transmissions are made without symbol preceding separate from the trellis coding procedure. The Reed-Solomon coding results are transmitted as 16-level (4 bits/symbol) one-dimensional-constellation symbol coding for cablecast, which transmissions are made without precoding. The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed.
The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 4-bit symbol coding have normalized values of -7, -5, -3, -1, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized vale of 1.25. The normalized value of +S is +5, and the normalized value of -S is -5.
The video carrier of an NTSC signal is 1.25 MHz from edge of the 6-MHz-wide broadcast channel, while the carrier for a digital TV signal for terrestrial through-the-air broadcast is 310 kHz from edge of the 6-MHz-wide broadcast channel. A co-channel NTSC signal does not exhibit symmetrical amplitude-modulation sidebands with respect to the carrier of the vestigial-sideband amplitude-modulation (VSB AM) carrying digital information. Accordingly, the NTSC video carrier at 940 kHz remove from digital TV signal carrier and its sidebands at further remove from digital TV signal carrier are not well canceled in the digital TV signal. Nor, of course, is the NTSC audio carrier at 5.44 MHz remove from digital TV signal carrier.
In the earlier development of the DTV art it was contemplated that the DTV broadcaster might be called upon to decide whether or not to use a symbol precoder at the transmitter, which symbol precoder would follow the symbol generation circuitry and provide for matched filtering of symbols, when used together with a comb filter in each DTV receiver used before the data-slicer in the symbol decoder circuitry as a symbol post-coder. This decision would have depended upon whether interference from a co-channel NTSC broadcasting station were expected or not. Symbol precoding would not have been used for data line synchronization code groups or during data lines in which data field synchronization data were transmitted. Co-channel interference is reduced at greater distances from the NTSC broadcasting station(s) and is more likely to occur when certain ionospheric conditions obtain, the summertime months during years of high solar activity being notorious for likelihood of co-channel interference. Such interference will not obtain if there are no co-channel NTSC broadcasting stations, of course. If there were likelihood of NTSC interference within his area of broadcast coverage, it was presumed that the HDTV broadcaster would use the symbol precoder to facilitate the IIDTV signal being more easily separated from NTSC interference; and, accordingly, a comb filter would be employed as symbol post-coder in the DTV receiver to complete matched filtering. If there were no possibility of NTSC interference or there were insubstantial likelihood thereof, in order that flat spectrum noise would be less likely to cause erroneous decisions as to symbol values in the trellis decoder, it was presumed that the DTV broadcaster would discontinue using the symbol precoder; and, accordingly, the symbol post-coder would then be disabled in each DTV receiver. A problem with using precoding at the transmitter is that, while preceding may be preferable insofar as certain DTV receivers are concerned, preceding may not be desirable for other DTV receivers receiving transmissions from the transmitter. DTV receivers less remote from the transmitter or more remote from a co-channel analog TV transmitter may not suffer significant amounts of co-channel interference, for example.
The Digital Television Standard the ATSC published Sep. 16, 1995 does not allow for the use of precoding of all data at the DTV transmitter to compensate for post-coding incidental to subsequent use of comb filtering in a DTV signal receiver to reject NTSC co-channel interference. Instead, only the initial symbol in the trellis decoding is precoded. This procedure by itself does not facilitate a DTV signal receiver using comb filtering to reject NTSC co-channel interference before data slicing procedures are undertaken. A DTV signal receiver that does not reject NTSC co-channel interference before data slicing procedures are undertaken will not have good reception under strong NTSC co-channel interference conditions as may be caused by the DTV receiver being remote from the DTV transmitter or having an analog TV transmitter very closeby.
In U.S. patent application Ser. No. 08/746,520 filed by the inventor on Nov. 12, 1996 , now U.S. Pat. No. 5,748,226 and entitled "DTV RECEIVER WITH FILTER IN I-F CIRCUITRY TO SUPPRESS FM SOUND CARRIER OF NTSC CO-CHANNEL INTERFERING SIGNAL", the inventor advocates preceding data-slicing in a DTV receiver with comb filtering to suppress NTSC co-channel interference when that interference is sufficiently large as to affect data-slicing adversely. The inventor teaches how to compensate in the symbol decoding procedure for the effects of such comb filtering upon symbol coding when it is selectively done. It is, then, still useful to be able to determine when NTSC co-channel interference is larger than a prescribed value denominated as being acceptably small, so that this determination can be used for controlling the selective use of comb filtering to suppress NTSC co-channel interference.
Comb filtering procedures are not entirely satisfactory for suppressing the NTSC audio carrier, particularly under conditions of frequency modulation in which carrier frequency deviation is large, since correlation (or anti-correlation) of samples of the FM carrier at times separated by any substantial fixed delay is apt not to be particularly good. U.S. patent application Ser. No. 08/746,520 advocates that the filtering used to establish the overall bandwidth of intermediate-frequency (IF) amplification be such as to reject the FM audio carrier of any co-channel interfering NTSC analog TV signal. Surface-acoustic-wave filtering in an ultra-high frequency (UHF) intermediate-frequency band is particularly suitable for rejecting the FM audio carrier of any co-channel interfering NTSC analog TV signal. Comb filtering of the baseband symbol codes is more satisfactory for suppressing the NTSC video carrier and nearby sidebands, which tend to exhibit good correlation between samples separated by certain specific delay intervals and to exhibit good anti-correlation between samples separated by other certain specific delay intervals. Suitable comb filtering procedures are also satisfactory for suppressing chroma sidebands near the NTSC chrominance subcarrier, which tend to exhibit good correlation between samples separated by certain specific delay intervals and to exhibit good anti-correlation between samples separated by other certain specific delay intervals. Suppressing the NTSC audio carrier during IF amplification allows comb filtering of the symbol codes with comb filters that reject NTSC video carrier and its chrominance subcarrier, but do not reject NTSC audio carrier very much. One such comb filter, which additively combines symbol codes having six-symbol-epochs differential delay, is favored since both the NTSC video carrier and its chrominance subcarrier tend to exhibit high degrees of self-anti-correlation over such a short differential delay.
U.S. Pat. No. 5,260,793 issued Nov. 9, 1993 to R. W. Citta et alii and entitled "RECEIVER POST CODER SELECTION CIRCUIT" selectively employs a post-coder comb filter for suppressing NTSC interference accompanying a real or in-phase baseband component (I channel) of the complex output signal of a demodulator used in a digital high-definition television (HDTV) receiver. The presence of NTSC interference in the I-channel component of the demodulator response is detected for developing control signals automatically to enable or disable the comb filter being used for suppressing NTSC co-channel interference. During each data field sync interval, the input signal to and the output signal from an NTSC suppression filter of comb filter type in the HDTV receiver are each compared with a respective signal that is known a priori and is drawn from memory within the HDTV receiver. If the minimum result of comparison with the input signal has less energy than the minimum result of comparison with the output signal from the NTSC suppression filter, this is indicative that the primary cause of variance from expected reception is random noise rather than NTSC co-channel interference. Insofar as the particular digital television receiver is concerned, reception would be better were precoding and post-coding not employed in the system, and it is presumed that the broadcaster has not employed precoding. If the minimum result of comparison with the input signal has more energy than the minimum result of comparison with the output signal from the NTSC suppression filter, this is indicative that the primary cause of variance from expected reception is NTSC co-channel interference rather than random noise. Insofar as the particular digital television receiver is concerned, reception would be better were precoding and post-coding employed in the system, and it is presumed that the broadcaster has employed precoding.
U. S. Pat. No. 5,546,132 issued August 13, 1996 to K. S. Kim et alii and entitled "NTSC INTERFERENCE DETECTOR" describes attempts to detect co-channel NTSC interference from data within the data segments other than those transmitting data field sync information. A comb filter that subtractively combines differentially delayed symbol coding in the I-channel is used at times it is necessary to suppress co-channel NTSC interference; and the use of a comb filter that additively combines differentially delayed symbol coding in the I-channel is advocated for extracting co-channel NTSC interference, so that the energy of the co-channel NTSC interference within a data segment can be determined. The problem with this approach is that the comb filter Kim et alii propose to use for extracting co-channel NTSC interference also has some response to symbol coding associated with the digital signal transmission. The energy associated with symbol coding can be higher than the energy of co-channel NTSC interference that will cause significant error in data slicing. This is particularly true when the DTV transmitter does not use precoding, so symbol code energy is concentrated at portions of the frequency spectrum that interleave with portions of the frequency spectrum in which NTSC signal energy is concentrated. Clearly, a better approach is required for determining, from data within the data segments other than those transmitting data field sync information, whether there are significant amounts of co-channel NTSC interference.
The Kim el alii apparatus can be improved by replacing the absolute value circuit they use after the comb filter used for extracting co-channel NTSC interference with a synchronous detector operable at the NTSC video carrier frequency of 59.75 times horizontal scan line rate, it is here pointed out. However, this introduces quite a bit of complexity into the Kim et alii apparatus. The advantages of synchronous detection of NTSC video carrier on being able to reject DTV modulation are better achieved in another way.
So long as analog television transmissions are made, causing NTSC co-channel interference to be a problem, DTV receivers will for commercial reasons probably be provided with auxiliary receiver apparatus for receiving analog TV broadcasting. Conventionally, receiver apparatus for receiving analog TV broadcasting uses what it called "intercarrier sound" in which an intercarrier-sound intermediate-frequency signal responsive to NTSC audio carrier is generated with 4.5 MHz carrier frequency, by mixing the amplitude-modulated NTSC video carrier and the frequency-modulated NTSC audio carrier as extracted from amplified very-high-frequency (VHF) intermediate-frequency signals. Generally, the filtering for extracting these carriers suppresses NTSC chrominance subcarrier by 10 dB or so compared to NTSC video carrier. The amplitude of the intercarrier-sound intermediate-frequency signal as generated responds to the amplitude of the amplitude-modulated NTSC video carrier. The intercarrier-sound IF signal is usually subjected to amplification that generates an amplified intercarrier-sound IF signal that is symmetrically limited on peaks of excursion, which amplified intercarrier-sound IF signal is supplied to a frequency-modulation detector used to reproduce baseband composite audio signal. Limiting the peaks of the amplified intercarrier-sound IF signal reduces the sensitivity of the FM detector to amplitude variations in the intercarrier-sound IF signal avoiding noise such as "sync buzz" attributable to the amplitude modulation of the NTSC video carrier.